Pulse width modulated pulsed DC arc welding

ABSTRACT

A method of arc welding is disclosed. Specifically, a method of pulsed DC arc welding is disclosed wherein the pulse width of the current pulses are modulated to maintain a constant time averaged power flow to the work pieces being welded. This method of arc welding is especially useful with a type of pulsed DC arc welding wherein the ratio of peak current to maintenance current is maintained at a selected high value and the current is cycled in a time duration whereby oxides on the surface of the work pieces are dissipated as the weld is made. When used with this type of pulsed DC arc welding the present method is especially suited for fluxless welding of aluminum work pieces, such as thin wall aluminum tubing used in making heat exchangers for air conditioning systems.

BACKGROUND OF THE INVENTION

This invention relates to arc welding and more particularly to pulseddirect current (DC) arc welding. Specifically, this invention relates toa method of pulsed DC arc welding wherein special pulse width modulatedpulses of positive direct current are used to weld together work pieces.

There are many situations in which it is desirable to arc weld togethertwo pieces of metal. For example, a heat exchanger for an airconditioning system may be made from sections of thin wall aluminumtubing which are joined to provide a continuous circuit for the flow ofa refrigerant. The sections must be joined so that there are no leaks.One method for accomplishing this is by arc welding.

One problem encountered in arc welding is the presence of foreignmaterials on the surfaces of the work pieces which are being weldedtogether. These foreign materials can degrade the quality of the weld ifthey are not removed. Metals such as aluminum, magnesium, and berylliumcopper, pose an especially difficult surface contaminant problem sinceoxides instantaneously form on the surfaces of these metals when theyare exposed to air. Oxides may be removed by using a nonmetal chlorineor fluorine base flux during the welding process but this flux iscorrosive and is not compatible with the environment. Therefore, it isdesirable to arc weld, especially to arc weld metals such as aluminum,magnesium, and berryllium copper without using a flux.

Fluxless welding is possible by using certain alternating current (AC)arc welding techniques. U.S. Pat. Nos. 3,894,210 to Smith, et al. and3,818,177 to Needham, et al. disclose such AC arc welding techniques.These techniques are especially useful for welding certain materials,such as aluminum, magnesium, and beryllium copper, since a weld can bemade even if oxides are present on the surfaces of the work pieces.However, there are many situations when it is desirable to use directcurrent (DC) arc welding. For example, it is difficult to weld thin wallsections of aluminum tubing used in making heat exchangers for airconditioning systems by using an AC arc welding technique. This isbecause AC arc welding requires a significant power flow to the workpieces to make a weld and dissipate oxides without using a flux. Thispower flow heats the work pieces to an undesirable temperature becausethe thin wall tubing does not provide a sufficient heat sink forconducting away heat energy. Thus, significant sagging in the weld areacan occur and there is a possibility that the work pieces will be burnedthrough. This distortion of the weld area can be reduced if DC arcwelding is used. Also, electrode life can be increased if DC arc weldingis used rather than AC arc welding. Furthermore, power flow to the workpieces may be more precisely controlled when using DC arc welding. Theseare just some of the advantages inherent in DC arc welding when weldingcertain materials such as the thin wall sections of aluminum tubing usedin making heat exchangers for air conditioning systems. Therefore, it ispreferable to weld these materials by using DC arc welding rather thanby using other techniques such as fluxless AC arc welding.

One disadvantage of conventional DC arc welding is that this type of arcwelding is not generally capable of fluxless welding of certainmaterials, such as aluminum, magnesium, and beryllium copper, which formdifficult to reduce oxides on their surfaces. However, there is a novelmethod of pulsed DC arc welding for welding these materials withoutusing a flux. This novel method is disclosed in copending U.S. patentapplication Ser. No. 252,567, filed Apr. 9, 1981, in the name of Moyeret al., entitled "Pulsed DC Arc Welding". This copending application isassigned to the same assignee as the present application.

According to this novel method, special pulses of positive directcurrent are applied at an arc gap to arc weld work pieces at the arcgap. The special pulses have a form which is similar to conventional DCpulses except that the ratio of the magnitude of the peak current to themagnitude of the maintenance current at the leading edge of each currentpulse is selected to have a special feature. Essentially, this ratio ismaximized and the increase from the maintenance current level to thepeak current value is adjusted to occur in a time interval whereby athermal shock effect is created. A related kind of thermal shock effectis well known in the field of vacuum brazing as part of a multi-stepheat treatment process in which materials are joined together bybrazing. Basically, this thermal shock effect results from rapidlyheating work pieces having surface oxides with a coefficient of thermalexpansion which is substantially less than the coefficient of thermalexpansion of the underlying pure material. The rapid heating causes anuneven rate of expansion which fractures and splits apart the oxides onthe surfaces of the work pieces.

The split apart oxides are pushed away from the weld area due to themelting and joining of the underlying pure materials during the novelarc welding process disclosed above. Other physical phenomena also maybe responsible for the exemplary welds formed when using this novel arcwelding method but the thermal shock effect is believed to be theprimary mechanism by which the oxides are dissipated. Regardless of theexact physical phenomena underlying the oxide dissipation, the featureof maximizing the ratio of peak current to maintenance current at theleading edge of each current pulse is an essential element of this novelmethod of DC arc welding. This feature is best explained when it isassumed that the thermal shock effect is the primary mechanism by whichthe oxides are dissipated.

The optimal values for the maintenance current, peak current and timeduration in which the increase from the maintenance current level topeak current value occurs, when arc welding according to the novel arcwelding method described above, are selected through a trial and errorprocess. These optimal values depend on the kind of material beingwelded, the thickness of the work pieces being welded, and other suchfactors.

Also, power flow from the welding electrode to the work pieces is animportant factor in determining weld quality. Good quality welds cannotalways be made because of changes in this power flow as a function oftime. It is especially difficult to continually make good quality weldson certain materials, such as thin wall aluminum tubing, when massproducing products, such as heat exchangers for air conditioningsystems, because of this variation in power flow. This problem ispresent even if the novel method of fluxless pulsed DC arc weldingdescribed above is used in the manufacturing process.

These changes in power flow usually are caused by variations in theresistance between the welding electrode and the work pieces due toinhomogeneities in the ionized gas, variations in work piece dimensionsresulting in a changing arc gap separation, naturally occurringfluctuations in power supply output voltage and other such phenomena.This variation in resistance between the welding electrode and the workpieces directly affects the amount of power which reaches the workpieces from the welding electrode. It is desirable to maintain thispower flow at a constant optimal value since it is this power flow whichprimarily determines weld quality.

Conventional arc welding systems of the pulsed DC type do notspecifically address the problem of controlling power flow to the workpieces. Typically, these systems regulate current flow by adjusting thevoltage applied across the arc gap in response to variations in arc gapresistance to maintain the current flow at constant preset levels.Therefore, the normal operation of a current regulated pulsed DC systemresults in variations in the power flow to the work pieces.

A method of controlling this power flow, when using a pulsed DC powersupply, is by changing the pulse width of the current pulses supplied tothe arc gap. If a periodic series of current pulses is being used thisamounts to changing the duty cycle of the current pulses. Thus, thismethod can be called pulse width modulation or duty cycle modulation.This method of controlling power flow is especially useful when the formof the DC pulses must be maintained as required when arc weldingaccording to the novel pulsed DC arc welding method described above.Therefore, it is desirable to have a method of pulsed DC arc weldingwhich is capable of precisely adjusting power flow to work pieces bypulse width modulating the current pulses without altering the generalform of the pulses. Furthermore, it is desirable to have a method of arcwelding which is capable of adjusting power flow to work pieces tocompensate for variations in the resistance between the weldingelectrode and the work pieces.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a methodof pulsed DC arc welding of work pieces wherein the pulse width of thecurrent pulses is modulated to precisely control the power flow to thework pieces.

It is another object of the present invention to provide a method ofpulsed DC arc welding of work pieces wherein the pulse width of thecurrent pulses is modulated to precisely control power flow to the workpieces without otherwise affecting the form of the current pulses.

It is a further object of the present invention to provide a method ofpulsed DC arc welding of work pieces wherein certain phenomena, such asvariable arc gap separation and fluctuations in power supply voltage,are automatically compensated for by the welding system.

It is a still further object of the present invention to provide amethod of pulsed DC arc welding of work pieces in which the power flowto the work pieces is controlled in response to variations in resistancesensed at the arc gap.

These and other objects of the present invention are accomplished byadjusting the pulse width of current pulses which are supplied to workpieces at an arc gap during arc welding. A voltage sensor is used tosense the voltage drop across the arc gap as a function of time. Thissensed voltage is directly proportional to the resistance across the arcgap and is a direct indicator of variations in variables affecting powerflow to the work pieces. The current pulses are adjusted in pulse widthto give a constant time-averaged power flow to the work pieces. Thus,when the resistance at the arc gap decreases the pulse width of thecurrent pulses supplied to the work piece at the arc gap is increasedwhile otherwise maintaining the form of the current pulses the same togive a constant time-averaged power flow to the work pieces.Alternatively, if the resistance at the gap increases the duration ofthe current pulses is decreased while otherwise maintaining the form ofthe current pulses the same to give a constant time-averaged power flowto the work pieces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of an arc welding system including afeedback control circuit 2 for adjusting the pulse width of currentpulses supplied by a power supply to an arc gap.

FIG. 2 is a schematic graph of the amplitude of current pulses which areapplied to work pieces to maintain the time-averaged power flow to thework pieces constant when the voltage drop sensed at the arc gap isincreasing.

FIG. 3 shows specific circuit components for the feedback controlcircuit shown in FIG. 1.

FIG. 4 shows a block diagram of an arc welding system including acontroller for automatically adjusting the pulse width of current pulsessupplied by a power supply to an arc gap.

FIG. 5 shows specific circuit components for the automatic controllershown in FIG. 4.

FIGS. 6 and 7 show how the impulsar output system shown in FIG. 5operates to generate output voltage control signals of two differentduty cycles in response to input voltage control signals of twodifferent magnitudes when the impulsar output system includes acomparator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a block diagram of an arc welding system isshown including a feedback control circuit 2 for modulating the pulsewidth of direct current (DC) pulses applied at an arc gap 3. The pulsesare modulated in pulse width while maintaining their peak magnitudeconstant to provide a constant time-averaged power flow across the arcgap 3. Current flow across the arc gap 3 from the electrode 12 to thework pieces 1 is determined by the operation of power supply 4. Thepower supply 4 can be one of a variety of power supplies which areavailable commercially. If the novel method of pulsed DC arc weldingdescribed previously is to be used it may be necessary to have a powersupply 4 with a relatively high peak current capability, depending onthe type of work pieces being welded, to provide the required ratio ofpeak current value to maintenance current level at the leading edge ofeach current pulse as required by this novel method. Conventional powersupplies may be modified by those of ordinary skill in the art toprovide a power supply with such a high peak current capability. Also,such a power supply is available from Creative Pathways, Inc., 2917Lomita Blvd., Torrance, Calif. 90505.

Impulsar 10 in connection with current regulator 11 controls theoperation of the power supply 4. This is a conventional type of controlfor a power supply 4. Also, a high voltage, high frequency arc starter 5controls the initial flow of current across the arc gap 3. This too is aconventional feature of arc welding systems. The arc starter 5 providesa high voltage to initiate current flow across the arc gap 3 by ionizinginert gas supplied to the arc gap 3 from the gas supply means 6 throughpassageways 14 in the electrode holder 15. After the initiation ofcurrent flow the arc starter 5 discontinues operation. Subsequently, theinert gas is ionized by the operation of the power supply 4 to sustaincurrent flow across the arc gap 3 throughout the arc welding process.The continuous supply of inert gas prevents impurities from reaching theweld and prevents formation of surface films, such as oxides, on thework pieces 1 during the arc welding process. However, it is notnecessary to supply inert gas during the welding process if other stepsare taken, such as providing a vacuum at the arc gap 2, to prevent oxideformation and impurities from reaching the weld.

The electrode holder 15 can be one of a variety of constructions. Forexample, the holder 15 can be a moving head type wherein the work pieces1 and the holder 15 are rotated relative to each other to effect weldingat selected positions on the work pieces 1. The holder 15 can beoperated to make a continuous weld on the work pieces 1 or a series ofspot welds.

A voltage sensor 7 senses the voltage drop across the arc gap 3 throughthe electrical leads 19 and 20. This voltage is directly portional tothe resistance across the arc gap 3. The voltage sensor 7 supplies anelectrical signal which indicates the resistance sensed at the arc gap 3to a high-low regulator 8. The high-low regulator 8 provides duty cyclesignal generator 9 with a control signal indicating whether the pulsewidth of the current pulses needs to be increased or decreased tomaintain a constant time-averaged power flow to the work pieces 1 at thearc gap 3. The high-low regulator 8 is designed so that a control signalis supplied to the duty cycle signal generator 9 only when the voltagesensed at the arc gap 3 by the voltage sensor 7 exceeds a preselectedhigh value or is below a preselected low value. The duty cycle generator9 supplies a continuous control signal to the impulsar 10 to result in apreselected baseline pulsed DC flow across the arc gap 3. However, whenthe duty cycle signal generator 9 receives a signal from the high-lowregulator 8 it responds to alter the operation of impulsar 10. The dutycycle signal generator 9 supplies a signal to the impulsar 10 toincrease the pulse width of the current pulses or decrease the pulsewidth of the current pulses depending on the control signal receivedfrom the high-low regulator 8.

It should be noted that the voltage sensor 7 is not the only type ofsensor which may be used to sense power flow related conditions at thearc gap 3. For example, a thin film resistance temperature detector(RTD) may be attached to the work pieces 1 to generate an electricalsignal which is a function of the temperature of the work pieces 1.Change in the temperature of the work pieces 1 are a reliable indicatorof variations in the power flow to the work pieces 1. The electricalsignal of the RTD device can be used to supply the high-low regulator 8with a voltage signal representing power flow which can be processed bythe high-low regulator 8 in the same manner as the electrical signalfrom the voltage sensor 7 is processed.

Referring now to FIG. 2, a schematic graph is shown of current pulsesvarying in pulse width as a function of time but having a constantperiod T_(o) between pulses. The positive DC pulses are preferably ofthe special novel type described previously wherein the leading edge ofeach current pulse is chosen to have a ratio of peak current tomaintenance current which is maximized to provide a thermal shock effectto dissipate oxides which may have formed on the surfaces of the workpieces 1. The difference in the thermal co-efficient of expansions of anoxide layer and the underlying pure metal results in the thermal shockeffect which dissipates the oxides. The present method of pulse widthmodulation is especially designed for this type of pulsed DC arcwelding.

The main principle of the present invention is the variation in pulsewidth to maintain power flow to the work pieces 1 constant for varyingresistances at the arc gap 3. The constant power flow to the work piecesimproves the quality of the weld. Sagging may occur if the weld is madeby supplying an excessive amount of power to the work pieces 1. Also,there is a possibility of burning through the work pieces if too muchpower is supplied to the work pieces 1. If too little power is suppliedto the work pieces 1 there may not be sufficient power to fullypenetrate the work pieces 1. A weaker and less durable weld resultscompared to when optimal power flow to the work piece is maintained. Thepresent invention reduces the possibility of poor weld quality by alwaysmaintaining optimal power flow to the work pieces.

For purposes of explanation, assume that a constant time-averaged powerflow P_(c) gives the optimal weld for a particular pulsed DC arc weldingprocess. The time-averaged power equation is: ##EQU1## where V(t) is thevoltage drop across the arc gap 3 as a function of time, I(t) is thecurrent flow across the arc gap as a function of time, and T is theperiod of the current pulses. Assume a constant period T_(o)corresponding to a fixed frequency F_(o), a constant voltage drop V_(o),and a periodically varying current flow given by the following functionrepeating itself during each successive period T_(o) : ##EQU2## whereI_(p) is a constant peak current value, where I_(m) is a constantmaintenance current value, which for purposes of this discussion can beassumed to be zero, and where X is the duty cycle of the current pulses.Assuming that I_(m) is zero, then integrating and solving the powerequation for X gives:

    X=P.sub.c /(V.sub.o I.sub.p)

This is the duty cycle necessary to sustain an optimal power flow P_(c)to the work pieces 1 at the arc gap 3, while maintaining a peak pulsedcurrent flow of I_(p), when the voltage drop across the arc gap 3 is aconstant V_(o).

If the voltage drop across the arc gap 3 increases to 2V_(o) thensolving for X gives:

    X=P.sub.c /(2V.sub.o I.sub.p)

Thus, the duty cycle must change to one-half the original duty cycle tosustain the optimal power flow P_(c) to the work pieces 1. If thevoltage drop increases to 4V_(o) then

    X=P.sub.c /(4V.sub.o I.sub.p)

and the duty cycle must change to one-quarter the original duty cycle tosustain this constant optimal power flow P_(c).

The above-described variation in duty cycle X is illustrated in FIG. 2where initially it has been assumed that a 50% duty cycle is required tosustain an optimal power flow P_(c) to the work pieces 1 at a constantvoltage drop V_(o) across the arc gap 3. A 50% duty cycle corresponds tocurrent flowing across the arc gap 3 for 50% of the operating time.During the other 50% of the operating time only the maintenance currentI_(m) flows across the arc gap 3. The second two pulses shown in FIG. 2represent the pulses generated when the voltage sensor 7 senses anincreased voltage drop at the arc gap 3 equal to 2V_(o). An increasedvoltage drop indicates an increase in resistance across the arc gap 3which means that the pulse width must be decreased to sustain the samepower flow P_(c) to the work pieces 1 at the arc gap 3. Thus, as shownby the second two current pulses the duty cycle is decreased to 25%. Thethird group of two pulses illustrates the variation in pulse width asthe voltage drop across the arc gap increases further to 4V_(o)indicating a further increase in resistance across the arc gap 3. Thepulse width is decreased to give a 12.5% duty cycle. Thus, although theinstantaneous power delivered to the work piece varies the time-averagedpower delivered to the work pieces is constant. Also, it should be notedthat the peak amplitude of the current pulses is maintained constant ata value I_(p).

FIG. 3 shows specific electrical components for the feedback controlcircuit 2 comprising voltage sensor 7, high-low regulator 8, and dutycycle signal generator 9 of the arc welding system shown in FIG. 1. Thevoltage sensor 7 comprises variable resistance device 21, resistor 22,and light emitting diode (LED) 23. Electrical leads 19 and 20 areconnected across the arc gap 3 as shown in FIG. 1. The variableresistance device 21 acts as a voltage divider to control the flow ofcurrent through resistor 22 and light emitting diode 23.

High-low regulator 8 comprises a variety of components includingphototransistor 29, op amp 30, op amp 32 and variable resistance devices34 and 35. Phototransistor 29 and capacitor 38, which are electricallyconnected in parallel to voltage supply 31, provide a voltage signal,representing the varying arc voltage which is sensed by the voltagesensor 7 and transferred to the phototransistor 29 from LED 23, to theinverting inputs of the op amps 30, 32. This representative voltagesignal is provided to the inverting input of op amp 30 through isolationresistor 39 and to the inverting input of op amp 32 through isolationresistor 40. This representative voltage signal is the input signalwhich is processed by the high-low regulator 8 to modulate the operationof the duty cycle generator 9. The phototransistor and capacitor 38 areconnected to ground through resistor 25. It should be noted that theterm "ground", when used in describing the high-low regulator 8 and theduty cycle generator 9, is equivalent to circuit common.

A voltage supply 33 supplies a reference voltage, which is adjusted byvariable resistance device 34, to the inverting input of op amp 30.Similarly, variable resistance device 35 adjusts the reference voltagesupplied by voltage supply 33 to provide an adjusted reference signal tothe inverting input of op amp 32. The signal from the variableresistance device 34 is provided to the op amp 30 through isolationresistor 36 and the signal from the variable resistance device 35 isprovided to op amp 32 through isolation resistor 37. These adjustedreference voltage signals are summed with the representative voltagesignal from the phototransistor 29 at the inverting inputs of the opamps 30 and 32.

The non-inverting inputs of the op amps 30 and 32 are connected toground through resistors 41 and 42, respectively. Variable resistancedevice 43 and capacitor 44 are connected in parallel to op amp 30 tocontrol the gain of the op amp 30. Also, time delay switch 45 isconnected in parallel to the op amp 30 to provide a shunting capabilityacross the op amp 30. Similarly for op amp 32, variable resistancedevice 46, time delay means 47 and capacitor 48 are connected inparallel to the op amp 32 for the same purposes. Diode 49, resistor 50and variable resistance device 52 are connected in series at the outputof op amp 30. Diode 49 blocks the transmission of negative outputvoltage signals from the op amp 30. Similarly, op amp 32 has diode 53,resistor 54 and variable resistance device 55 connected in series at theoutput of op amp 32. Diode 53 blocks positive output voltage signalsfrom the op amp 32. It should be noted that the variable resistancedevices 52 and 55 control the magnitude of the voltage signals outputtedfrom the op amps 30 and 32, respectively. The voltage signals from theop amps 30 and 32 are summed at point 56 and supplied to the duty cyclesignal generator 9 shown in FIG. 1.

The duty cycle signal generator 9 comprises a voltage divider includingvariable resistance device 57 with resistor 60 and variable resistancedevice 58 with resistor 61. Also, the generator 9 includes op amp 59. Avoltage supply 62 supplies voltage to the variable resistance devices 57and 58. A time delay switch 63 is connected in parallel to the variableresistance device 58 to provide a means for shunting variable resistancedevice 58. The output from the voltage divider is supplied throughresistor 64 to the inverting input of op amp 59. This voltage signal issummed with the voltage signal from the high-low regulator 9. Thenon-inverting input of op amp 59 is connected to ground through resistor65. The gain of op amp 59 is controlled by variable resistance device 66and capacitor 68. The output from op amp 59 is supplied to impulsar 10,as shown in FIG. 1, through lead 69. The voltage divider insures that areference signal is always generated by op amp 59 for supply to impulsar10. If a signal is present at point 56 it is summed with the output fromthe voltage divider to provide an adjusted input signal to the op amp 59which adjusts the output signal from op amp 59.

In operation, voltage is sensed at the arc gap 3 through leads 19 and 20of voltage sensor 7. This voltage signal is adjusted by the variableresistance device 21 and supplied through the resistor 22 to the lightemitting diode (LED) 23. The light emitting diode 23 emits light havingan intensity which varies in direct proportion to the voltage sensed atthe arc gap 3. Thus, a higher sensed voltage causes the LED 23 to emitlight of a greater intensity.

The phototransistor 29 detects the intensity of the light from the LED23. This interaction of the phototransistor 29 and the LED 23 occurswithin an area 24 of the feedback control circuit 2, as shown in FIG. 3.The phototransistor 29 is utilized to electrically isolate the high-lowregulator 8 from the voltage sensor 7 thereby isolating the arc weldingpower supply 4 from the high-low regulator 8. The relatively slowresponse speed of the phototransistor 29 eliminates undesirable spuriouselectrical interference from being picked up by the high-low regulator8. Capacitor 38 and phototransistor 29 function to create arepresentative voltage signal from the variable voltage signal which issensed at the arc gap 3 by the voltage sensor 7 and which is transferredto the phototransistor 29 from the LED 23. The phototransistor 29 ispowered by a voltage supply 31.

The representative voltage signal from the phototransistor 29 is summedat the inverting inputs of the op amps 30 and 32 with an adjustedreference voltage from voltage supply 33. The reference voltage suppliedto op amp 30 is adjusted by variable resistance device 34 and thereference voltage supplied to op am 32 is adjusted by variableresistance device 35. These reference voltage signals are adjusted tocancel particular representative voltage signals from thephototransistor 29 which are generated when particular preselectedvoltages are sensed at the arc gap 3. The selected voltages are a highvoltage and a low voltage corresponding to a high power flow level tothe work pieces 1 and a low power flow level to the work pieces 1,respectively. The high power flow level is a power flow level above theoptimal time-averaged power flow level and the low power flow level is apower flow level below this optimal level. The high and low power flowlevels are limits beyond which it is undesirable to have thetime-averaged power flow deviate if optimal welding is to be achieved.The high and low voltages may be selected to equal each other if nodeviation from optimal time-averaged power flow is to be tolerated.However, usually some deviation is allowed to prevent the high-lowregulator 8 from constantly modulating the duty cycle of the currentpulses supplied across the arc gap 3 to the work pieces 1.

For example, if a representative voltage signal from phototransistor 29to the inverting input of op amp 30 of minus 2.5 volts occurs at theselected high voltage signal corresponding to a high power flow levelwhich it is desired not to exceed, then variable resistance device 34 isset to that a plus 2.5 volts is supplied from the voltage supply 33 tothis inverting input of op amp 30. A positive output voltage signal,which is allowed to pass to the duty cycle generator 9 by diode 49,appears from the op amp 30 only when the phototransistor 29 provides arepresentative voltage signal to the inverting input of op amp 30 ofless than minus 2.5 volts. The amount by which this representativevoltage signal is less than the minus 2.5 volts corresponds to theamount by which the duty cycle of the current pulses at the arc gap 3must be decreased to maintain the optimal time-averaged power flow tothe work pieces 1. Alternatively, if a minus 1.5 volts representativesignal occurs at the selected low voltage signal corresponding to a lowpower flow level which it is desired not to fall below, then variableresistance device 35 is set so that a plus 1.5 volts is supplied fromthe voltage supply 33 to the inverting input of op amp 32. A negativeoutput voltage signal, which is allowed to pass to the duty cyclegenerator 9 by diode 53, appears from the op amp 32 only when thephototransistor 29 provides a representative voltage signal to theinverting input of op amp 32 of greater than minus 1.5 volts. The amountby which this representative voltage signal is greater than the minus1.5 volts corresponds to the amount by which the duty cycle of thecurrent pulses at the arc gap 3 must be increased to maintain theoptimal time-averaged power flow to the work pieces 1.

Thus, the op amps 30, 32 and diodes 49, 53 operate to provide an outputvoltage signal only when the voltage sensed at the arc gap 7 andtransmitted to the phototransistor 29 exceeds certain limits which areset by the variable resistance devices 34 and 35. If the variation inthe voltage at the arc gap 3 does not exceed one of these preset limitsthan no voltage signal is outputted from the op amps 30 and 32 to poing56. However, if the voltage sensed should exceed either of thepreselected limits then a voltage proportional to the amount by whichthe voltage exceeds the limit is outputted from either op amp 30 or opamp 32. The magnitude of this output voltage signal is adjusted by theresistance devices 50, 52 for the op amp 30 and by the resistancedevices 54, 55 for the op amp 32. This adjusted voltage signal issupplied to the inverting input of the op amp 59.

The voltage divider provides a continuous signal to the inverting inputof op amp 59. Thus, if no voltage signal is supplied from op amps 30 and32 to the op amp 59 the op amp 59 will still have an outputcorresponding to the signal supplied at its inverting input from thevoltage divider. If there is a signal present at the point 56, thissignal is summed with the signal from the voltage divider to provide analtered signal at the inverting input of op amp 59. The voltage signalat point 56 varies depending on the voltage sensed at the arc gap 3 bythe voltage sensor 7. The signal from op amp 59 is transmitted throughthe lead 69 to the impulsar 10 of the arc welding system.

Time delay switches 45, 47 and 63 are used to prevent the feedbackcontrol circuit 2 from improperly operating during the start-up periodfor the arc welding system. Initially, switches 45 and 57 are closed andswitch 63 is open, as shown in FIG. 3. When switches 45 and 47 areclosed op amps 30 and 32, respectively, are shunted and therebyprevented from operating. If the op amp 30 was allowed to operate whenthe arc started 5 is ionizing the inert gas at the arc gap 3 andinitiating current flow across the arc gap 3, then it would defeat theoperation of the arc starter 5. After the arc starter 5 has completedits function and after a first preselected time delay the normally openswitch 63 closes and the normally closed switch 47 opens. During thisfirst preselected time delay the current pulses supplied at the arc gap3 have a larger duty cycle (pulse width) than desired for steady-stateoperation of the welding system. This larger duty cycle, during thisfirst time delay, insures that proper heat transfer, fusion, andpenetration is occurring at the work pieces 1 during the start-upperiod. When normally open switch 63 closes the variable resistancedevice 58 is shunted thereby lowering the voltage signal which isprovided to op amp 59 from the voltage divider circuit. This alters thevoltage control signal outputted from op amp 59 to step-down the dutycycle of the current pulses supplied to the arc gap 3. The duty cycle ofthe current pulses is decreased to the duty cycle desired forsteady-state operation, that is, to that duty cycle which has previouslybeen determined to result in optimal power flow to the work pieces 1.Normally closed switch 47 opens at the same time that normally openswitch 63 closes thereby enabling op amp 32 to provide low limitregulation of the current pulses. Thus, if the voltage drop across thearc gap 3 is not sufficient to achieve optimal power flow at thedecreased duty cycle then the op amp 32 operates to increase the dutycycle of the current pulses to compensate for this deficiency.

After a second preselected time delay, after step-down, the switch 45opens to enable op amp 30 to provide high limit regulation of thecurrent pulses. Op amp 30 is not enabled until after step-down toprevent the op amp 30 from interfering with arc stabilization during thestart-up period. The opening of switch 45 ends the start-up period.Also, this allows the high-low regulator 8, through the operation of opamps 30 and 32, to modulate the duty cycle control signal outputted byop amp 59 of the duty cycle signal generator 9, as described previously.

The impulsar 10 of a conventional arc welding system commonly includes acircuit having a comparator for generating a control signal for thecurrent regulator 11. Thus, the level of the voltage signal supplied bythe duty cycle signal generator 9 to the impulsar 10 determines the dutycycle of the current pulses supplied by the power supply 4 to the arcgap 3. As the magnitude of the voltage signal from the op amp 59 of theduty cycle generator 9 increases the duty cycle of the pulses suppliedat the arc gap 3 increases.

This is accomplished by supplying the voltage signal from the duty cyclesignal generator 9 to the comparator of the impulsar 10 and thenproperly processing the output voltage signal from this comparator. Anexample of the operation of such a comparator is explained in thefollowing discussion of FIGS. 6 and 7. It should be noted that there aremany techniques of utilizing the voltage signal supplied from the dutycycle signal generator 9 to adjust the pulse width of the current pulsessupplied at the arc gap 3. Also, it should be noted that the selectedtechnique depends on the construction of the particular impulsar 10which is being used. The foregoing is only one such technique for animpulsar which includes a particular comparator circuit.

Referring now to FIG. 4, a block diagram is shown for an arc weldingsystem having a time delay programmable pulse width controller 80 usedas part of the control system for the power supply of the arc weldingsystem. As shown in FIG. 4, work pieces 70 and an electrode 71 form anarc gap 73 across which a voltage is supplied by power supply 74. Highfrequency high voltage arc starter 75 is also connected across the arcgap 73 to provide an initial high voltage for ionizing the inert gassupplied at the arc gap 73 from the gas supply means 76 at the beginningof a welding cycle and for initiating current flow across the arc gap73. After this initial ionization and after the initial current flowbegins the arc starter 75 discontinues operation. The power supply 74 isa commercially available pulsed positive DC power supply. A conventionalcurrent regulator 77 controlled by a conventional impulsar is used tocontrol the operation of the power supply 74.

As shown in FIG. 4, the impulsar is depicted as divided into two parts.One part is designated an impulsar pulse width adjustor 79 and the otheris designated an impulsar output system 78. The impulsar pulse widthadjustor 79 is that part of the conventional impulsar which generatesinternal control signals for the impulsar, typically voltage signals,for controlling operation of electrical devices of the impulsar.Typically, the magnitude of an internal voltage control signal 101supplied through an electrical lead 81 from the pulse width adjustor 79to the impulsar output system 78 determines what pulse width controlsignal 104 will be outputted from the impulsar, as depicted in FIGS. 6and 7. Other internal control signals may flow from the impulsar pulsewidth adjustor 79 to the impulsar output system 78 via electricalconnector 82.

The impulsar output system 78 is that part of the conventional impulsarwhich generates an output control signal 104 for the current regulator77 in response to the internal voltage control signal 101 from theimpulsar pulse width adjustor 79. Typically, the impulsar output system78 includes a comparator which compares a reference voltage signal 102,such as a voltage ramp function, to the internal voltage control signal101 from the pulse width adjustor 79. FIGS. 6 and 7 depict how thecomparator operates to generate output voltage signals 103 of differentpulse widths in response to internal voltage control signals 101 ofdifferent magnitudes. Basically, the comparator generates an outputvoltage signal 103 only when the reference voltage signal 102 equals orexceeds the internal voltage control signal 101. Thus, the internalvoltage control signal 101 shown in FIG. 6 results in a smaller pulsewidth for the comparator output voltage signal 103 compared to when thereduced internal voltage control signal 101 shown in FIG. 7 is utilized.Other conventional circuit elements of the impulsar output system 78respond to the comparator output voltage signal 103 to generate animpulsar output control signal 104 for the current regulator 77. Thisoutput control signal 104 is keyed to the off-times of the comparatoroutput voltage signal 103 so that the duty cycle of the current pulsessupplied by the power supply 74 to the arc gap 73 is keyed to theoff-times of the comparator output voltage signal 103. Thus, an increasein the internal voltage control signal, 101, which causes a decrease inthe pulse width of the comparator output voltage signal 103, results ina corresponding increase in the duty cycle of the current pulsessupplied at the arc gap 73.

The impulsar pulse width adjustor 79 of the arc welding system directlycontrols the operation of the impulsar output system 78. However,according to the principles of the present invention, a time delayprogrammable pulse width controller 80 is interposed between theconventional pulse width adjustor 79 of the impulsar and the impulsaroutput system 78. This programmable pulse width controller 80 operatesto automatically adjust the impulsar output system 78 to control thecurrent regulator 77, and thus power supply 74, to provide currentpulses of varying pulse width according to a predetermined programmedsequence.

The programmable pulse width controller 80 can be of a variety ofconstructions. The controller 80 can be most simply constructed byproviding a variable resistance device 89 and a time delay switch 88connected in parallel to each other and in series between the pulsewidth adjustor 79 and the impulsar output system 78. Such a controller80 is shown in FIG. 5. It should be noted that the circuit shown in FIG.5 is a simple example of a programmable pulse width controller 80. Othercircuits could be devised by one of ordinary skill in the art to provideother more complex current programs.

In operation, positive DC pulses are supplied at the arc gap 73 by thepower supply 74 as controlled by the current regulator 77 in response tothe input from the impulsar output system 78. Initially a signal issupplied through the normally closed contacts of the time delay switch88. However, time delay switch 88 operates after a preselected timedelay to open the normally closed contacts. This interposes the variableresistance device 89 between the impulsar pulse width adjustor 79 andthe impulsar output system 78. This results in a different signal beingprovided to the current regulator 77 and in turn to the power supply 74.This different signal adjusts the duty cycle of the pulses supplied atthe arc gap 73. Typically, the duty cycle of the current pulses isdecreased after the time delay. Normally, a decrease is required sincethere is a heat build-up at the work pieces 70 during the start-upperiod of operation of the arc welding system. Thus, it is necessary toreduce the power flow to the work pieces 70 after a period of time tomaintain the optimal power flow which will consistently achieve goodquality welds. The particular time delay and amount of reduction in dutycycle to achieve optimal welding depends on the particular work pieces70 being welded. These parameters are best selected through a trial anderror process.

Finally, it should be noted that, although the pulse width modulation ofDC pulses according to the principles of the present invention isparticularly suited for welding materials, such as aluminum, when usingthe special novel type of current pulse described previously, thepresent invention is not limited to use with this type of pulse. Pulsewidth modulation according to the principles of the present inventionprovides precise control of power flow to work pieces at an arc gap whenarc welding practically any kind of material with DC pulses. Forexample, conventional DC pulses used in welding together stainless steelwork pieces, especially thin wall pieces of stainless steel, can bemodulated according to the principles of the present invention toprovide precise control of the power flow to the work pieces to makehigh quality welds. Therefore, while the present invention has beendescribed in connection with particular embodiments, it is to beunderstood that various other embodiments and modifications may be madewithout departing from the scope of the invention heretofore describedand claimed in the appended claims.

What is claimed is:
 1. A method of arc welding of work pieces whichcomprises:positioning an electrode and the work pieces relative to eachother to form an arc gap; providing inert gas continuously at the arcgap; applying across the arc gap an arc starter voltage having amagnitude sufficient to ionize the inert gas and initiate current flowacross the arc gap; discontinuing the arc starter voltage; providing amaintenance current flow across the arc gap which is sufficient tosustain a minimum current flow across the arc gap throughout the arcwelding process, said maintenance current providing a power flow whichis insufficient to increase the temperature of the work pieces to themelting temperature of the work pieces; increasing the magnitude of thecurrent flowing across the arc gap to a peak value which can providesufficient power flow to melt the work pieces and which is of sufficientmagnitude that a power flow is provided which is capable of dissipatingoxides on the surfaces of the work pieces during the time interval inwhich the increase in current flow occurs; holding the current flowacross the arc gap at substantially the increased value for a durationof time sufficient to provide enough energy to heat the work pieces totheir melting temperature; decreasing the magnitude of the currentflowing across the arc gap to substantially the maintenance currentvalue to allow the temperature of the work pieces to decrease to atemperature below their melting temperature whereby the work pieces arewelded together; periodically cycling the current flow across the arcgap by repeating the steps of increasing, holding and decreasing thecurrent flow to vary the magnitude of the current flowing across the arcgap between the maintenance current value and the peak current value toform a periodic series of current pulses which are applied to the workpieces; continuously sensing the resistance at the arc gap; adjustingthe duration of time at which the current flow across the arc gap isheld at substantially the increased value in response to the resistancesensed at the arc gap to provide a constant time-averaged power flow tothe pieces; and changing the relative position of the electrode and thework pieces to direct each current pulse to a selected porton of thework pieces.
 2. The method as recited in claim 1 wherein the work piecesare aluminum.
 3. The method as recited in claim 2 wherein the ratio ofthe magnitude of the peak current to the magnitude of the maintenancecurrent is at least 7.5.
 4. The method as recited in claim 3 wherein theperiodic series of current pulses has a frequency between 1 and 50 hertzwith a duty cycle of 10 to 20%.
 5. The method as recited in claim 1wherein the relative position of the electrode and the work pieces ischanged to direct the current pulses to overlapping portions of the workpieces to form a continuous weld on the work pieces.
 6. The method asrecited in claim 1 wherein the relative position of the electrode andthe work pieces is changed to direct the current pulses to distinctportions of the work pieces to form a series of spot welds on the workpieces.